Many polymeric materials are foamed to provide low density articles such as films, cups, food trays, decorative ribbons, and furniture parts. For example, polystyrene beads containing low boiling hydrocarbons such as pentane are formed into lightweight foamed cups for hot drinks such as coffee, tea, hot chocolate, and the like. Polypropylene can be extruded in the presence of blowing agents such as nitrogen or carbon dioxide gas to provide decorative films and ribbons for package wrappings. Also, polypropylene can be injection molded in the presence of these blowing agents to form lightweight furniture parts such as table legs and to form lightweight chairs.
Polyesters such as poly(ethylene terephthalate) have a much higher density (e.g. about 1.3 g/cc) than other polymers. Therefore, it would be desirable to be able to foam polyester materials to decrease the weight of molded parts, films, sheets, food trays, and the like. Such foamed articles also have better insulating properties than unfoamed parts. However, it is difficult to foam such polyester materials because of the low melt viscosity and low melt strength of typical poly(ethylene terephthalate) and related polyester polymers. The low melt viscosity and low melt strength of the polyesters is a problem because the polymer melt will not adequately retain the bubbles of an expanding gas. It would be desirable therefore to be able to provide polyester polymers which could be foamed with conventional foaming systems.
One approach to provide polyesters with high melt strength involves treating preformed polyesters with multifunctional carboxylic acids or polyols to provide branched polyesters. Such compositions are disclosed in U.S. Pat. Nos. 4,132,707; 4,145,466; 4,999,388; 5,000,991; 5,110,844; 5,128,383; and 5,134,028. The branching agents used include tri- and tetracarboxylic acids and anhydrides such as trimesic acid, pyromellitic acid, and pyromellitic dianhydride or polyols such as trimethylolpropane and pentaerythritol. These branching agents will provide polyesters with increased melt viscosities and melt strengths but their use is often disadvantageous. For one thing, the branching agent cannot be put into the initial reaction mixture of polyester reagents because this will lead to crosslinked structures. If added after the polyester is formed, an additional processing step is required and the branching action is hard to control. The usual method of adding the branching agent is to melt the polyester in an extruder and to add the branching agent into the melt in the barrel of the extruder. It is difficult to control the amount of branching agent used and to get adequate mixing and reaction before the melt exits the die. It would be advantageous therefore to provide a method whereby the branching agent and possibly other additives could be dry blended with the polyester pellets prior to the extrusion operation.
Other patent references of interest with regard to foaming polymers include U.S. Pat. Nos. 3,879,505; 4,149,485; 4,176,101; 4,761,256; 4,284,596; 4,351,911; 4,462,947; 4,466,933; 4,473,665; 4,626,183; 4,728,673; 4,734,304; 4,737,523; 4,746,478; 4,751,250; 4,981,631; and 5,182,307.
The closest prior art of which we are aware concerning polyester/polyolefin blends include U.S. Pat. Nos. 4,981,631, 3,960,807, 4,572,852 and 5,229,432 but none of these patents are related to a master batch concept for incorporating some other agents into a polyester composition.
It has now been found that branching agents such as trimellitic acid, pyromellitic dianhydride and the like can be melt compounded with polyolefin polymers to provide concentrates. These concentrates can be dry blended with polyesters such as poly(ethylene terephthalate), dried in vacuum or conventional ovens, and then extruded and foamed with conventional inert gases, volatile organic compounds, or chemical blowing agents. During the melt compounding, the branching agent reacts with the polyester to provide branching and thus to increase the melt viscosity and the melt strength of the polyester. The effect of this is to significantly improve the foamability and blow molding characteristics of the polyester. The polyolefin of the concentrate component is also beneficial for improving the impact properties of the polyester.